With increasing incidence, allergic disease is a significant source of morbidity in adults and children. The pathogenesis of allergic disease requires that immunoglobulin (Ig) E (IgE) molecules be produced against otherwise innocuous substances. Despite the essential role of IgE in allergic disease, there is a fundamental gap in understanding how IgE is produced and regulated from its source-namely, IgE B cells. Continued existence of this gap represents an important problem because, until it is filled, the pathogenesis of IgE- mediated diseases such as asthma, food allergies and allergic rhinoconjunctivitis will remain largely incomprehensible. The long-term goal is to uncover regulatory mechanisms of allergic disease and other environmental intolerances so as to identify new targets that can be manipulated for therapeutic purposes. The objective in this particular application is to deploy unique mouse models to determine how B cell function and fate is influenced by IgE expression in the context of the B cell receptor (BCR) from its natural genomic context-and to begin to translate the findings to human IgE B cells. The central hypothesis is that IgE as a BCR directs a program for B cell survival and function that is distinct from the other IgH isotypes due to its unique signaling as a BCR. This hypothesis has been formulated on the basis of preliminary data using an IgE mouse model cloned from an IgH isotype-switched B cell. The rationale for the proposed research is that once it is known how IgE functions as a BCR, new targets for manipulation of the IgE response may be generated. This hypothesis will be tested by pursuing three specific aims: 1) Discover how BCR signaling is influenced in the context of IgE; 2) Determine the consequences of IgE expression on B cell fate and survival; and 3) Determine the role of antigen-specific IgE on cognate interactions and downstream developmental potential compared to other IgH isotypes specific to the same antigen. Under the first aim, unique mouse models producing B cells with pre-switched IgH isotypes, as well as IgE, IgM, IgA and IgG1 B cell lines, will be deployed to uncover the unique molecular, biochemical, and cellular pathways downstream of antigen engagement of IgE as a BCR. Under the second aim, these tools will be utilized to examine the extent to which IgE B cells can particulate in somatic hypermutation and Ig class switch recombination (CSR). In the third aim, novel antigen-specific IgE mice will be used to identify the degree to which IgE B cells may process and provide antigen to cognate T cells and recruit limiting T cell help. The proposed work is innovative, because it represents a departure from the status quo by providing unique models to examine IgE B cell biology wherein membrane and secreted IgE are produced from natural genomic contexts. The proposed work is significant because it is a key step in a continuum of research that is expected to uncover new targets for preventive and therapeutic interventions at the IgE B cell stage. Ultimately, such knowledge has the potential to inform the development of new strategies that will help to reduce the growing problem of allergic disease in the U.S.